This invention relates to a magnetron, more particularly to a magnetron of the type wherein permanent magnets are contained in an evacuated envelope.
The magnetron of this type is shown for example by FIG. 2 of the papers entitled "A Hitachi's New Magnetron with an Inserted Magnetic Circuit" which was prepared and reported by Inchiro Inamura et al at the International Microwave Power Institute Symposium of May 26, 1977. With this type of the magnetron as it is possible to dispose the permanent magnet close to the interaction space defined between the cathode electrode and the anode vanes, the leakage of the flux of the magnetic path is decreased thus increasing the utilization factor of the flux. Accordingly it is possible to miniaturize the magnet and magnetron. For this reason, this type is considered important.
In this type of the magnetron, for the purpose of preventing diffusion of the electrons emitted from the cathode filament into the interaction space end shields are mounted on the opposite ends of the filament and the diameter of the end shields is made a little smaller than the inner diameter of the vanes. On the other hand, when the inner diameters of the ring shaped magnet and the pole piece thereof through which the support of the cathode structure constituted by the filament and the end shields extend is made small as far as possible, it is possible to make uniform the field distribution in the interaction space so that the inner diameters of the pole piece and the magnet are generally made smaller than the outer diameter of the end shield.
The input side of such magnetron is assembled by arranging a yoke around a support which is secured to a stem for holding the cathode structure, mounting on the yoke split halves of the magnet and pole piece in this order, inserting these magnet and pole piece halves while holding them by the aid of a suitable holding member, and finally welding the yoke to the anode cylinder.
The permanent magnet made of such a brittle material as samarium-cobalt is formed to a ring shape prior to assembling and split into halves for its arrangement around the support, so that no shoulder or stepped portion between the two halves is formed after completion of assembling. The pole piece made of iron, however, has difficulties with its separation into two halves, which is easy for the magnet as mentioned above, and therefore two halves of the pole piece are manufactured independently, i.e., not through the separation of single pole piece. This leads to dimensional errors of the independently manufactured two halves of the pole piece and a consequent shoulder between them even if they are positioned with accuracy. More particularly, the magnet and the pole piece are disposed between a yoke, one component element of the envelope, and the holder which is secured to the inner wall of the envelope. The holder is provided with an opening larger than the projection of the pole piece extending through the opening so that by the engagement of the projection and the inner wall of the opening, the flat surface of the pole piece is caused to contact the opposing surface of the holder. If the above described gap were formed they do not contact each other uniformly. Such imperfect contact increases resistance to the flow of surface current which results in concentration of electric field thus causing spark and local heating. Consequently, gas is released to damage the heated cathode filament. Also, the generation of the aforementioned spark has its heat which is transmitted to the magnet, thereby causing the magnet to be heated above 300.degree. C. As above described the magnet is generally made of a sintered samarium-cobalt so that when the magnet is heated to a temperature of higher than 300.degree. C., its magnetromotive force is lost.